The propeller has been known to be arranged vertically adjustably in watercraft intended for transportation in shallow waters. During travel in water of sufficient depth, the propeller operates in a depth of water that is optimal from the viewpoint of the overall situation, while the propeller is adjusted in the upward direction during travel in correspondingly shallow water to the extent that it is still able to bring about propulsion, but contact with the bottom is ruled out, and when the watercraft is in extremely shallow waters, the propeller is even adjusted in the upward direction to the extent that it is located above the deepest point of the hull.
The so-called rudder propellers are a special type of boat propulsion. In the case of rudder propellers, the propeller is used not only to generate the propulsive force needed for moving the boat, but it is also pivotable such that the direction of the action of the propulsive force, i.e., not only the speed of travel, but also the direction of travel can be determined.
The following design has been known if a rudder propeller unit is used in a watercraft that is intended for transportation in shallow waters. In view of the small depth of water, the hull has a flat, level bottom. A shaft-like housing ("container") led out of the interior of the boat opens in the plane of this flat boat's bottom, and the propeller is mounted adjustably in the axial direction (in the longitudinal direction of the housing) in this housing. To cause the boat to travel in a defined direction, the propeller is located under the plane of the boat's bottom outside the shaft-like housing, and it is driven by a drive shafting, on the top end of which a drive motor acts, and whose lower end acts on the propeller, and which is led through this housing coaxially to the shaft-like housing. When the boat is in extremely shallow waters, the propeller is adjusted in the upward direction in relation to the shaft-like housing, and into the housing, if necessary. By rotating the propeller around the longitudinal axis of the drive shafting, it is possible to preset the direction of travel for the movement of the boat.
Even though the present invention does not pertain exclusively to such a propulsion, problems of such a propulsion are mainly discussed preferably and in connection with the explanation of the present invention.
If the rudder propeller, the drive shafting, and the drive motor at the top end of the drive shafting form, in their entirety, an assembly that is adjustable in relation to the shaft-like housing in a rudder propeller unit with propeller adjustable in the longitudinal direction of the drive shafting, all kinds of problems occur, which develop when large masses are to be adjustable: High adjusting forces are to be applied, and the accuracy of adjustment to be required can be achieved only with considerable effort. In addition, a relatively large free space in the hull is necessary in rudder propeller units, and this free space must be available above the motor when the unit is in its lower end position, and into which the unit is adjusted, especially with the motor, when the unit is to be adjusted in the upward direction from its lower end position, and this free space is lost as a useful space for boat cargo or the like. Even though this problem is alleviated when the motor is permanently installed in the hull and the drive shafting is designed as a longitudinally adjustable drive shafting by nonrotatably connecting, e.g., a motor-side drive shaft part and a propeller-side drive shaft part via spline shaft teeth. However, this solution also requires a relatively long and correspondingly heavy shaft-like housing, whose length is determined by the maximum length of the longitudinally adjustable drive shafting, namely, when the propeller has been maximally extended from the housing, i.e., when the maximum distance is present between the motor and the propeller.